Method of making composite pressure vessels



June 28, 1966 M. A. KRENZKE METHOD OF MAKING COMPOSITE PRESSURE VESSELSOriginal Filed Aug. 27, 1964 4 Sheets-Sheet 2 Lu [I D (I) (I) u.! 0: O.

i l l J O O O O O O O N N 1 (D (.0 O l l l T INVENTOR. NOISNELLNOISSBHdl/QOO MARTIN A. KRENZKE ATTY.

June 28, 1966 M. A. KRENZKE 3,257,718

METHOD OF MAKING COMPOSITE PRESSURE VESSELS Original Filed Aug. 27, 19644 SheetsSheet 5 FIG. 4.

I NVENTOR.

MART A. KRENZKE BY g/l I ATTY.

June 28, 1965 M. A. KRENZKE 3,

METHOD OF MAKING COMPOSITE PRESSURE VESSELS Original Filed Aug. 27, 19644 Sheets-Sheet 4 United States Patent States of America as representedby the Secretary of the Navy ('hriginal application Aug. 27, 1964, Ser.No. 392,662. Divided and this application Apr. 23, 1965, Ser. No.

2 Claims. (Cl. 29404) The invention described herein may be manufacturedand used by or for the Government of the United States of America forgovernmental purposes without the payment of any royalties thereon ortherefor.

This application is a division of patent application Serial No. 392,662,now abandoned, filed August 27, 1964, which in turn is acontinuation-in-part of application Serial No. 860,297, filed December17, 1959.

The present invention relates to a method of making a pressure vessel.

In undersea operation, particularly at great depth, large hydrostaticpressures are encountered. For this reason in the past, submarinepressure hulls have been generally constructed of welded steel. Thereare two great disadvantages of construction of pressure vessels whenhigh pressures are involved. First, the shell thickness becomes verylarge and thus the rolling and welding required in fabrication areextremely diflicult. Secondly, the weight of the structure is oftengreater than desired. It has been proposed to use light high-strengthmaterials such as titanium and aluminum and alloys thereof.

The unique pressure vessel made by the method of the present inventionis a composite hollow body composed of an inner hollow body constitutedof a plurality of physically unattached blocks abutting each other andheld in place solely by means of a jacket surrounding the blocks andwhich has been placed in a state of residual tension.

In the construction of the pressure vessel made by the method of thisinvention a composite hollow body is formed of an external jacket inwhich are placed a plurality of physically unattached blocks or segmentswhich may have high strength-to-weight ratios or other desired physicalcharacteristics. As part of the composite hollow body the jacketpron/ides longitudinal strength to resist bending movements. The jacketalso provided watertight integrity and resistance to corrosion. Thefunction of the blocks or segments is to resist the major portion ofcompressive forces due to the pressure load, such as that due toexternal hydrastatic pressure. examples the shape of the compositehollow body may be spherical, cylindrical, oblong, irregular, or anysuitable shape depending upon the use intended.

In fabricating the composite body acording to one version of theinvention, assurnig that the desired shape is, for purposes ofillustratio only, cylindrical, a plurality of rings or segments thereofare ararnged in an unconnected stacked fashion in close fit with theinside of a jacket. The ends of the jacket are sealed in any suitablemanner, as for example, by hemispherical bodies. The resulting hollowbody is then subjected to increasing external pressures, for example, bysubmergence in water.

During initial stages of pressure build-up, the jacket is elasticallydeformed inwardly, and if there is no initial gap between jacket andinner segments, the jacket and By way of g I 3,257,718 Patented June 281966 ice inner segments share in resisting the radial load due to theapplied external pressure. As long as there is a gap, the jacket resistsall radial loading. Eventually the applied pressu-re reaches a point atwhich the elastic limit of the jacket is exceeded, and the jacketmaterial undergoes plastic flow and, neglecting strain-hardening,resists no additional load.

Concurrently, the aditional load on the composite body due to externalpressure is no longer taken up by the jacket but is taken up by theinner segments, which are elastically deformed inwardly.

After reaching a predetermined pressure loading which is less thancollapse-pressure, the external pressure is then released. During thisdecompression phase, the materials of the inner segments and jacket acttogether. The particular manner in which the jacket and segments coactdepends on the compatibility of the respective materials. By the termcompatibility is meant the relation between yield points and moduli ofelasticity of the jacket and segment materials and the eflect of thisrelation when the jacket and segments are, subjected to pressure, thendecompression.

As one example, reference is made to the generalized explanatory graphsof FIGS. 1, 2 and 3, each of which shows a plot of Y-axis stress versusX-axis static pressure acting on the jacket (curve 1) and the innersegments (curve 2) respectively. In the example of FIG. 1, the jacket(which may be steel) has a higher modulus of elasticity than thesegments (which may be titanium alloy) while the segments have a higheryield point. It is also assumed that there is no clearance or a slightclearance, between the jacket and segments, but not enough clearance toenable the Baushinger efiect to take place.

As shown in FIG. 1, the jacket (curve 1) takes up al of the radialloading until point A, which is reached at a relatively low appliedexternal pressure, and additional loads are shared by the jacket andinner segments. As the jacket reaches its yield point at B, and betweenat least points B and C, further pressure is applied, the jacketmaterial transmits all additional pressure load to the segment material.

Upon release of the applied pressure (point C), the jacket materialreturns along curve 1 which is steeper than return curve 2 because ofthe jackets higher modulus of elasticity.

Meanwhile, the segment material which never did reach its yield pointalong curve 2, returns elastically upon decompression (as indicated bythe arrow along its return curve 2).,

At Zero applied pressure, the jacket material is in a state of residualtension while the segments are in a state of residual compression. Thismeans that the jacket holds the segments together. The essentially sameresult may be obtained if the jacket is preshrunk on the segments.

Suppose, however, that the jacket and segment mate rials have the samemodulus of elasticity, such as, for example, in the case where atitanium jacket encloses segments of high strength titanium. An exampleof how such materials may be compatible is shown in the graph of FIG. 2.The FIG. 2 is a plot of the same quantities as FIG. 1.

Assuming no initial clearance between jacket and segments, as thepressure is initially applied, both the jacket and the segments followthe same elastic deformation 3 curve (jacket curve 1 superimposed onsegment curve 2). The jacket passes its yield point at point A andcontinues to yield at the same stress as the pressure is increased, butthe segments are stressed at an increased rate because they are nowcarrying all the additional load.

At point B the pressure is released and the stress in jacket andsegments follows respective pressure stress curves 1 and 2. At zeropressure there is residual tension in the jacket and slight residualcompression in the segments. These residual forces coact so that thejacket holds the segments tightly together. The composite body acts toresist both bending moments and compressive forces.

As yet another example of compatibility, the graph of FIG. 3 illustratesthe situation where the jacket material has a yield strength and modulusof elasticity greater than those of the inner segments.

As an illustration but not a limitation, the jacket material in such acase may be steel and the material of the inner segments may bealuminum. In some respects, FIG. 3 resembles FIG. 1, except that in FIG.3 curve 1 representing stress on the jacket indicates a greater amountof external pressure that is applied to the jacket before the innersegments take up all of the additional pressure loading at yield pointA. Sufficient additional loading (from point A to point B) places theinner segments in sutficient elastic compression so that when thepressure is released, the jacket (having the higher modulus ofelasticity) returns along steeper curve 1 to a point at zero appliedpressure of residual tension which holds the inner segments tightlytogether in residual compression.

From the foregoing examples of the present invention it can beappreciated that the jacket and segment materials may be regarded ascompatible when during the composite body-forming cycle of compressionof the jacket and segments, the jacket plastically flows while thesegments are elastically deformed to such an extent that upon release ofthe pressure, the jacket and segment materials are resultantly inresidual tension and compression respectively. It is not necessary thatthe respective jacket and segment materials be different or that theyhave different moduli of elasticity or different yield strengths.

It is, therefore, an object of the present invention to provide a novelcomposite pressure vessel in which physical attachment of the elementsthereof prior to treatment is unnecessary.

Another object of the present invention is to provide a type and methodof pressure vessel construction which permits the use of non-weldablematerials which are not practical to machine.

It is another object of the present invention to eliminate the necessityof welding thick sections in constructing pressure vessels.

A further object is a pressure hull construction which permits the useof assembly line methods to thereby reduce time as well as cost ofconstruction.

Still another object of the present invention is the provision of amethod of fabrication of pressure vessels which have high strength toweight ratios.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the same becomes better understood byreference to the following detailed description when considered inconnection with the accompanying drawings in which like referencenumerals designate like parts throughout the figures thereof andwherein:

FIGS. 1, 2 and 3 are graphs previously referred to for explaining theprinciples of the present invention.

FIG. 4 is a sectional view taken along the axis of the hull of anoceanographic research vessel made in accordance with one version of thepresent invention; and

FIG. 5 is a view partly in section of one of the internal hull segmentsshown in FIG. 1, and

FIG. 6 is a view in perspective of a vessel made in accordance withanother version of the present invention.

Referring now to the drawings, wherein like reference charactersdesignate like or corresponding parts throughout the views, there isshown in FIG. 4 a hull made of jacket and inner segment materials havingrespectively diiferent yield strength and Youngs moduli of elasticity.The hull of FIG. 4 is composed of a jacket 11 which is cylindrical inshape and may be made of rolled and welded sheets of metal. Insidecylindrical jacket 11 are a plurality of ring segments 12 shown indetail in FIG. 4 which are arranged adjacent one another along theentire length of the jacket. The segments 12 may be held in place bygravity alone. At each end of the cylinder is fitted a dome 13 whichforms the ends of the pressure vessel.

The vessel hull shown in FIGS. 4 and 5 is designed for use as anoceanographic research vessel for exploration of the ocean bottom atgreat depths. There is provided an access hatch 14 in one of the domes13. A cover 16 is fittedover and preferably hinged to the hatch 14 andis used to provide access to the interior of the hull. Cover 16 may bemade of transparent material to allow observation. It will be realizedthat a plurality of transparent observation windows may be provided inthe domes.

It will also be realized that holes may be provided in the domes forsteering mechanism and propulsion shafts. The construction of theseopenings is conventional and forms no part of the present invention.

In constructing the pressure hull shown in FIG. 4 the first step isforming the jacket 11 into cylindrical form then welding the sheetstogether, as can be seen in the figure the jacket is relatively thinmaterial so that welding is greatly facilitated. As an illustration, andnot a limitation, the material used for the jacket may be a steel havingyield strength with approximately 80,000 pounds per square inch. Thissteel has a value of Young's modulus of approximately 30x10 pounds persquare inch.

Inside the steel jacket are placed in end to end relationship a seriesof aluminum rings. The rings are not physically attached to each other.These rings may be of either machined, cast, or rolled construction.Aluminum having a yield strength of approximately 70,000 pounds persquare inch with the Youngs modulus of approximately 10x10 pounds persquare inch has been found compatible for use with a steel jacket alongthe lines exemplarily illustrated in FIG. 3.

Dome shaped ends 13, preferably hemispherical in shape and which may beprovided with access and observation hatches and propeller shaftopenings are fitted onto the ends of the cylindrical structure incontact with the ends of the outer two rings of the aluminum and arepermanently welded to the steel jacket. As shown in FIG. 4 the ends areof solid steel construction. However it will be realized that these toomay be of aluminum abutting the segments and a steel jacket covering thealuminum and welded to the cylindrical steel jacket.

When all of the rings are in place and the ends are attached to thesteel jacket in bearing relationship to the rings, the device is subjectto large external pressures by submergence under water. The pressure isregulated so that the steel outer jacket is caused to be stressed beyondits yield point but the yield point of the inner segments has not yetbeen reached. The pressure is then removed and as this initial load isreleased the predetermined difference in Youngs modulus and yieldstrength establishes the condition in which the aluminum tends to expandmore than the steel and therefore puts the steel jacket in residual orprestressed tension. This prestressing locks the aluminum segments inplace event when no external loads are applied. It will be realized thatthe thickness of the inner segments depends upon the amount of pressurefor which the vessel is being designed. Webs 17 and flanges 18 may beprovided for added structural strength without the ad dition of largeamounts of additional weight.

Instead of forming the segments with a flange and web as shown in thefigures, it may be desirable in some instances to use segments ofconstant thickness in abutting relationship with an I-beam shapedsupport abutting the inside of the joint along the length thereof.Segments of different shapes in overlapping relationships might also beused for this purpose. In this manner only half of the segments need tobe machined or cast, the other half consisting merely of rolled stock.The domes 13 bear against the ends of the outer aluminum sections andtend to compress them thus holding them longitudinally in place.

Reference is now made to FIG. 6 in which inner segments in the form ofcurved blocks 20 having inwardly tapering sides are arranged inside acylindrical jacket 21. Each of the blocks 20 abut the jacket 21 and theblocks abut each other. As shown in FIG. 5 the block may be arranged inbrick-like fashion, each course thereof defining a ring of abutting butphysically unattached segments.

It is to be understood that FIG. 6 is only exemplary and that a jacketof any suitable shape may be employed in conjunction with blocks ofvarying or constant size shaped to abuttingly fit therein to define ahollow body to be formed into a composite unitary body in accordancewith the invention. For example, an ellipsoid jacket may be providedwith an inner body formed of abutting blocks of different sizes andshapes; a spherical body may have geodesic blocks or circular rows ofblocks of the same size and shape in each row, but differing in size andshape from row to row with a large lid type block at one end. The blocksmay be stacked in igloo fashion that is,'overlappingly. Also, plasticmaterial may be inserted between the blocks forachieving better fit andfor minimizing the chance of cracking in the blocks when pressureloaded.

It will be realized that although the invention has been described inrelation to certain named materials, various other materials may be usedas desired. For example, a steel jacket may be used in conjunction withsteel inner segments. Or, a metal jacket (e.-g. steel, titanium) may beemployed with glass or ceramic or glass reinforced plastic innersegments. It is not necessary that the jacket be of a weldable materialbecause jackets made by other methods such as forging and extruding maybe used.

If the difference in Yoiings modulus and yield strength of the materialsused is sufficient to allow construction as previously described thevessel may be constructed in that manner. However, if the initialconditions, and the physical characteristics of the materials are suchthat fabrication of the device in this manner is not practical,

shrink-fit methods of fabrication may be used wherein the outer shell orjacket 11 is heated and segments 12 having a slightly larger externaldiameter than the inner diameter of the cold jacket are placed insidethe jacket while the jacket is heated. The device is then allowed tocool causing contraction of the outer jacket and the segments are heldin place with the jacket being in tension as previously described. Thistype of construction could be used, for example, when a high strengthnon-weldable steel segment is to be placed inside a weldable steeljacket. The shrink-fit type of construction may also be accomplished insome instances merely by the contraction of the weld as it cools.

It is understood that the blocks or segments composing the inner hollowbody are held together at essentially atmospheric pressures by thetension of the jacket acting thereon. When the composite body issubjected to higher external pressure, such as a pressure which exerts aforce on the inner body greater than the force imposed thereon by thepre-tensioned jacket, the force due to that higher external pressureserves to act on the inner body to hold the blocks or segments thereoftogether. Where blocks are used to form the inner hollow body, it is, ofcourse necessary that the blocks be inwardly tapered so that each blockacts as a keystone.

Although the present invention has been described for use in eliminatingthe necessity of welding the basic structural elements it will berealized that the invention may also be utilized to provide a protectivecovering Where corrosion or other deterioration is involved for example,a jacket of material which does not corrode may be placed overstructural elements which if exposed to sea water might deteriorate in ashort time. It should be under: stood therefore that the invention isnot limited to an oceanographic research vessel but may also be used invarious other devices which are required to withstand external forces.It will also be realized that various materials may be used in theconstruction. Since many modifications and variations of the presentinvention are possible in the light of the above teachings, it istherefore to be understood that within the scope of the appended claimsthe invention may be practiced otherwise than as specifically described.

What is claimed is:

'1. A method ofmaking a fluid-tight composite pressure vessel forwithstanding bending moments and high external pressures, comprising thesteps of:

forming a substantially cylindrical open-ended jacket member of weldablematerial having a preselected yield point and modulus of elasticity;

placing a plurality of physically unattached body elements arranged inside-by-side relationship within said jacket member to form a hollowbody member therein capable of resisting high compressive loads andbeing made of a material having a pre-selected yield point and modulusof elasticity;

attaching dome-shaped members to the open ends of said jacket member tothereby seal the open ends of said jacket member;

submerging said composite vessel in water to a depth where thehydrostatic pressure applied thereto reaches the yield point of thejacket material;

submerging said composite vessel in water to a predetermined additionaldepth where the hydrostatic pressure applied thereto exceeds the yieldpoint of said jacket material with said jacket material therebyundergoing a predetermined plastic deformation and said material of saidinner body member undergoing a predetermined elastic deformation; and

returning said composite vessel to the surface of the water to a regionof essentially atmospheric pressure, at which pressure the jacketmaterial is in a 'state of residual tension and the material of the bodymember is in a state of residual compression;

the residual tension in the material of said jacket member beingoperable to hold the jacket member and body member in r-igid assembly.

2. A method of making a fluid-tight composite pressure vessel forwithstanding bending moments and high external pressures, comprising thesteps of:

forming a substantially cylindrical open-ended jacket member of weldablematerial having a pre-selected yield point and modulus of elasticity;

placing a plurality of physically unattached body elements arranged inside-by-side relationship within said jacket member to form a hollowbody member therein capable of resisting high compressive loads andbeing made of a material having a predetermined yield point and modulusof elasticity;

inserting dome shaped members into the open ends of said jacket memberto a point where each dome shapedmember is in contact with one of saidbody elements;

welding the -.dome shaped members to the ends of the acket member tothereby seal the ends of said jacket member;

submerging said composite vessel in water to a depth where thehydrostatic pressure applied thereto reaches the yield point of thejacket material;

submerging said composite Nessel in water to a predetermined additionaldepth where the hydrostatic pressure applied thereto exceeds the yieldpoint of said jacket material With said jacket material therebyundergoing a predetermined plastic deformation and said material of saidinner body member undergoing a predetermined elastic deformation; [andreturning said composite vessel to the surface of the Water to a regionof essentially atmospheric pressure,

at Which pressure the jacket material is in -a state of 10 residualtension and the material of the body member is in a state of residualcompression;

1. A METHOD OF MAKING A FLUID-TIGHT COMPOSITE PRESSURE VESSELWITHSTANDING BENDING MOMENTS AND HIGH EXTERNAL PRESSURES, COMPRISING THESTEPS, OF: FORMINGL A SUBSTANTIALLY CYLINDRICAL OPEN-ENDED JACKET MEMBEROF WELDABLE MATERIAL HAING A PRE-SELECTED YIELD POINT AND MODULUS OFELASTICITY; PLACING A PLURALITY OF PHYSICALLY UNATTACHED BODY ELEMENTSARRANGED IN SIDE-BY-SIDE RELATIONSHIP WITHIN SAID JACKET MEMBER TO FORMA HOLLOW BODY MEMBER THEREIN CAPABLE OF RESISTING HIGH COMPRESSIVE LOADSAND BEING MADE OF A MATERIAL HAVING A PRE-SELECTED YIELD POINT ANDMODULUS OF ELASTICITY; ATTACHING DOME-SHAPED MEMBERS TO THE OPEN ENDS OFSAID JACKET MEMBER TO THEREBY SEAL THE OPEN ENDS OF SAID JACKET MEMBER;SUBMERGING SAID COMPOSITE VESSEL IN WATER TO A DEPTH WHERE THEHYDROSTATIC PRESSURE APPLIED THERETO REACHES THE YIELD POINT OF THEJACKET MATERIAL; SUBMERGING SAID COMPOSITE VESSEL IN WATER TO APREDETERMINED ADDITIONAL DEPTH WHERE THE HYDROSTATIC PRESSURE APPLIEDTHERETO EXCEEDS THE YIELD POINT OF SAID JACKET MATERIAL WITH SAID JACKETMATERIAL WHEREBY UNDERGOING A PREDETERMINED PLATIC DEFORMATION AND SAIDMATERIAL OF SAID INNER BODY CHAMBER UNDERGOING A PREDETERMINED ELASTICDEFORMATION: AND RETURNING SAID COMPOSITE VESSEL TO THE SURFACE OF THEWATER TO A REGION OF ESSENTIALLY ATMOSPHERIC PRESSURE, AT WHICH PRESSURETHE JACKET MATERIAL IS IN A STATE OF RESIDUAL TENSION AND THE MATERIALOF THE BODY MEMBER IS IN A STATE OF RESIDUAL COMPRESSION; THE RESIDUALTENSION IN THE MATERIAL OF SAID JACKET MEMBER BEING OPERABLE TO HOLD THEJACKET MEMBER AND BODY MEMBE IN RIGID ASSEMBLY.